Today, a large variety of electronic portable devices is available, for example mobile devices such as mobile telephones, personal navigation devices, but also (sub-) notebooks, ultra-mobile PCs, or peripheral devices for connection to a PC such as external network cards or external storage components. Handheld devices may be seen as a class of portable devices comprising, besides mobile telephones, also smartphones, wireless e-mail clients, personal digital assistants (PDAs), etc. In general, each of these portable devices comprises one or more printed circuit boards (PCBs), which may be arranged in one or more PCB layers extending parallel to each other to enable the typical, essentially flat or plane shape of many portable devices. Each PCB supports a plurality of electronic components generating substantial amount of heat, for example microprocessors, power supply components, FPGAs (Field Programmable Gate Arrays) and DSPs (Digital Signal Processors).
As a general trend, portable devices tend to contain more and more functionalities. The functionalities may comprise new, more and/or more complex applications. As an example, following the evolution of mobile network technologies, mobile devices are developed which transmit or receive data with increasingly higher data rates. With increasing complexity, generally also the power consumption of the portable devices increases. As the size of portable devices is essentially constant or even shrinking, the problem of cooling the devices becomes more and more important, i.e. the problem of keeping the temperature inside the device below a maximum allowed temperature.
For larger, stationary electronic devices, such as personal computers (PCs), various cooling mechanisms are conventionally applied. For example, heat sinks may be applied to PCBs or directly to particularly heat-generating electronic components such as microprocessors, ASICs (Application Specific Integrated Circuits) or transistors. Heat sinks, which may be made from metal with high thermal conductivity such as aluminium, are typically dimensioned as required for dissipating the thermal energy from the corresponding electronic components and may, for example, be massive components and/or comprise spacious ribs for efficient dissipation. Heat sinks may thus be much larger than the electronic components on a PCB, which generally is not problematic because there are no strict requirements on the dimensions of, for example, a PC housing. In other cases, if an overly large heat sink is required, this may be mounted to the outside of a stationary device, for example in the field of power electronics.
As a further cooling technique, a fluid such as ambient air may be used as a heat dissipating medium. In many cases in which passive air cooling is not sufficient, fans are used as airflow generators in order to generate an internal airflow which increases heat dissipation from components such as transformers or hard drives. In a typical configuration, a PC housing may comprise one or more holes acting as air inlets, one or more holes acting as air outlets and an internal fan adjacent to, e.g., the air outlet.
The cooling problem for portable devices cannot be solved by simply transferring the cooling concepts from PCs or other stationary devices. While there typically is enough space available in larger devices, for example for providing sufficiently large fans or heat sinks, portable devices are limited in internal space.